Painted reconstruction by Brian Choo.
When: Devonian (~390 to 360 million years ago)
Where: Europe and North America
What: Cheirolepis is one of the oldest of the ray-finned fishes. It was about ~10 inches (25 cm) long on average. It was covered with thick scales that were in tight articulation with each other with an even heavier covering on the head. This ‘head shield’ was made of jointed dermal bones. Cheirolepis had a heteroceral tail, meaning the bones forming the tail were not in the middle of the structure. The tail appears symmetrical however due to the well developed fin. Its pectoral fins were also covered with jointed dermal bones. Cheirolepis had well developed teeth, both on the margins of its mouth on its jaw bones, but also deeper inside its mouth on interior bones. It would have been able to open its mouth extremely wide; preying on animals up to half its size. It was a fast swimming predator of the devonian seas, not even the heavily armored placoderms were safe - the teeth on the inner skull bones would have been well suited for crunching their armor plating.
The ray finnied fishes, the Actinopterygii, are today by far the most common fish in the world. Cheirolepis is a Devonian actinopterygian, but the first members of the group date to the Silurian. Only about ten genera of ray-finned fishes are known from the Devonian, and all look very simular to Cheirolepis. The fish that dominated the devonian waters were the lobefins: the Sarcopterygii. As you have probably gathered, a major difference between these forms even in the ancient Paleozoic is how the fin is formed. Actinopterygians have a simple serial arrangement of bones all in a row from the back of the fin to the front. Whereas, Sacopterygians have much sturdier bones in their fins, connecting to each other down the length of the fin.
In this picture the Sarcopterygiifin is on the left and the Actinopterygii on the right.
After the devonian the ray finned fishes had a dramatic explosive radiation. Even today though there are some living ray-fins that look a lot like Cheirolepis, such as Amia - the bowfin. Today lobe fined fish are extremely rare, using a non cladistic definition. However there are more sacopterygians around than you might first think! Tetropods are descendant from these lobe finned fishes. As humans are tetrapods, you too are a Sacopterygian!